C# coalesce operator Throws

Question

I have a class with a string property. I use the coalesce operator when reading from it as it might be null, but it still throws me an NullRefrenceExeption.

string name = user.Section.ParentSection.Name ?? string.Empty;

To be more specific, its the ".ParentSection" that's null so is it because it don't even have ".name" ? If that's the case should i test ".ParentSection" first with an if block?

I assume there are something about the Coalesce operator i dont understand, hope someone can shed some light on whats going wrong here.

As the answers imply, the null coalescing operator is not throwing here; it's the evaluation of its left operand that throws the exception. — phoog, Mar 28 '12 at 14:56

score 7 · Accepted Answer · answered Mar 28 '12 at 14:49

7

To be more specific, its the ".ParentSection" that's null so is it because it don't even have ".name" ?

Yes.

If that's the case should i test ".ParentSection" first with an if block?

Yes.

answered Mar 28 '12 at 14:49

Jakub Konecki

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Thats what i assumed, but i tough it might be an more elegant solution to it. Thanks :) – Lasse Vabe Rolstad Mar 28 '12 at 14:58
There is a more elegant solution called the Maybe Monad. See my answer. – FishBasketGordo Mar 28 '12 at 15:00

score 4 · Answer 2 · answered Mar 28 '12 at 14:59

You'll need to check if Section and ParentSection are null. You could use an if-statement for this or write an extension method like this:

public static class MaybeMonad
{
    public static TOut With<TIn, TOut>(this TIn input, Func<TIn, TOut> evaluator)
        where TIn : class
        where TOut : class
    {
        if (input == null)
        {
            return null;
        }
        else
        {
            return evaluator(input);
        }
    }
}

You would use this method like so:

string name = user.With(u => u.Section)
                  .With(s => s.ParentSection)
                  .With(p => p.Name) ?? string.Empty;

I think it's a lot cleaner than an if-statement with a lot of &&.

Some further reading: http://www.codeproject.com/Articles/109026/Chained-null-checks-and-the-Maybe-monad

score 3 · Answer 3 · answered Mar 28 '12 at 14:50

3

You need to check if user, user.Section, or user.Section.ParentSection are null before you can use the null coalescing operator on a property of user.Section.ParentSection.

answered Mar 28 '12 at 14:50

Justin Rusbatch

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score 3 · Answer 4 · answered Mar 28 '12 at 14:50

Nested property access is not safe if any of the objects accessed are null this will throw a NullReferenceException. You will have to explicitly test for the outer objects to be not null.

E.g.:

string name = string.Empty;
if(user!=null && user.Section!=null && user.Section.ParentSection !=null)
   name = user.Section.ParentSection.Name ?? string.Empty;

In general I would try to avoid nested access to properties, you are violating the Law of Demeter. Some refactoring might make this unnecessary in the first place.

I would agree; `public String getParentSectionName()` would be a welcomed change I'm sure. — Brad Christie, Mar 28 '12 at 14:56

score 2 · Answer 5 · answered Mar 28 '12 at 14:52

The ?? operator checks if the left side is null and if so returns the right one, if not the left one. In your case the left-side is the "Name" property in the object user.Section.ParentSection and this is null.

In those cases either think on what might be null or do something like this:

string name = user == null 
              || user.Section == null 
              || user.ParentSection == null 
              || user.Section.ParentSection.Name == null 
                 ? string.Empty 
                 : user.Section.ParentSection.Name;

(yeah it's ugly I know)

score 1 · Answer 6 · answered Mar 28 '12 at 14:50

Chances are user or user.Section or user.Section.ParentSection is a null value.

The ?? operator doesn't prevent checks like:

if (user != null && user.Section != null && user.Section.ParentSection != null){

Make sure that everything up to the string property is valid and exists, then you can use ??. You can't call (null).Name, no matter how many times you try.

score 0 · Answer 7 · answered Mar 28 '12 at 14:53

0

Yes you need to check if Section or ParentSection are null before you check Name

answered Mar 28 '12 at 14:53

ABH

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score 0 · Answer 8 · answered Mar 28 '12 at 14:53

0

It is probably best to do something like this:

if(user!=null && user.Section!=null && user.Section.ParentSection!=null)
{
     string name = user.Section.ParentSection.Name ?? string.Empty;
}

answered Mar 28 '12 at 14:53

Arion

31,011
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score 0 · Answer 9 · answered Mar 28 '12 at 15:04

The null coalescing operator takes a statement like:

a = b ?? c;

What this says is "evaluate b; if it has a non-null value then assign that to a. Otherwise assign the value of c to a".

However within your b you're using a user object which may be null that has a section object that may be null that has a parent section property that may be null that had a name property that may be null. If you wanted to check all of these (and typically you should) then you can do something like:

string name = string.Empty;  
if (user != null &&  
     user.Section != null &&  
     user.Section.ParentSection != null)  
{  
  name = user.Section.ParentSection.Name ?? string.Empty;  
}

As soon as the IF check fails it will not check further and therefore you don't get a NullReferenceException when you assume an object is present and then try to access one of its properties.

C# coalesce operator Throws

9 Answers9